add HostMap.RemoteIndexes (#329)

This change adds an index based on HostInfo.remoteIndexId. This allows
us to use HostMap.QueryReverseIndex without having to loop over all
entries in the map (this can be a bottleneck under high traffic
lighthouses).

Without this patch, a high traffic lighthouse server receiving recv_error
packets and lots of handshakes, cpu pprof trace can look like this:

      flat  flat%   sum%        cum   cum%
    2000ms 32.26% 32.26%     3040ms 49.03%  github.com/slackhq/nebula.(*HostMap).QueryReverseIndex
     870ms 14.03% 46.29%     1060ms 17.10%  runtime.mapiternext

Which shows 50% of total cpu time is being spent in QueryReverseIndex.

handshake.go

@@ -30,7 +30,6 @@ func HandleIncomingHandshake(f *Interface, addr *udpAddr, packet []byte, h *Head
 	}
 
 	if tearDown && newHostinfo != nil {
-		f.handshakeManager.DeleteIndex(newHostinfo.localIndexId)
-		f.handshakeManager.DeleteVpnIP(newHostinfo.hostId)
+		f.handshakeManager.DeleteHostInfo(newHostinfo)
 	}
 }